The invention generally relates to a method for operating a turbine and to a turbine installation.
In industrial installations, for example in installations for generating electricity, a gaseous medium is supplied to a turbine in order to drive it. As a rule, the turbine is connected to a generator for generating electrical energy or, for example, it drives a compressor or a pump. In the case of a steam turbine, the gaseous medium is live steam. Before it is supplied to the turbine, this live steam is heated in a boiler, which is connected upstream of the turbine.
A method and an appliance for recording and evaluating undesirable temperature and pressure changes is described in D1U.S. Pat. No. 4,655,041 (DEL VECCHIO RICHARD J ET AL) Apr. 7, 1987 (1987-04-07). The temperature and the pressure of the steam flowing in a turbine are cyclically measured and compared with specified values. If the specified values are exceeded, an alarm occurs.
A first alarm occurs if the temperature becomes greater than a specified maximum temperature or less than a specified minimum temperature or the pressure becomes greater than a maximum pressure or less than a minimum pressure. The permissible temperature range is located between the maximum temperature and the minimum temperature. The permissible pressure range is located between a maximum pressure and a minimum pressure.
A second alarm occurs if the temperature and the pressure are outside the permissible temperature range or the pressure range, and the temperature change and/or pressure change exceeds a previously determined value.
A third alarm occurs if the temperature and the pressure are outside the permissible temperature range or the pressure range, and the temperature change and/or pressure change exceed, a specified maximum value.
An object is achieved, according to an embodiment of the invention, by a method for operating a turbine, in particular a steam turbine to which a gaseous medium is supplied, wherein a change with time of the temperature of the medium is monitored.
The monitoring of the temperature change, i.e. the observation of the variation in the temperature gradient, may be based on the consideration that an excessively rapid temperature changexe2x80x94even if it lies within the permitted temperature range between the absolute limiting valuesxe2x80x94can lead to turbine damage. This is because in the case of an excessively rapid temperature change, or on the occurrence of temperature steps, material problems occur, under certain circumstances, which have a disadvantageous effect, particularly on the efficiency of the turbine. Further, they may lead, under certain circumstances, to cracks and to fracture of the material. Compared with conventional methods, which only monitor whether the temperature exceeds a specified absolute limiting value, this achieves a clearly improved protective function.
The monitoring of the temperature change therefore opens the possibility of having already taken appropriate preventive measures in the case of an excessively large or an excessively rapid change in temperature.
When a maximum temperature gradient, as a measure for the change with time of the temperature, is exceeded, the supply of the medium to the turbine is preferably interrupted by executing a rapid shut-down. Consequently, the method permits a certain value of the temperature change. If this value is exceeded, in particular for a longer period, the supply of the live steam is interrupted in order to protect the turbine from an excessively large thermal stress.
In a preferred embodiment, the maximum permissible temperature gradient is specified as a function of the load condition of the turbine and particularly, in fact, in such a way that the maximum permissible temperature gradient becomes smaller with increasing load. This is based on the consideration that in the case of low load conditions, the heat transfer from the live steam to the material of the turbine is small, in particular because of the low density and the low velocity of the live steam. In consequence, higher temperature gradients are permitted in the low-load range without the danger of turbine damage occurring.
In addition to the monitoring of the temperature change, the supply of the medium to the turbine may be expediently interrupted when an absolute limiting value for the temperature is exceeded. A permissible absolute temperature range, within which the live steam temperature can vary, may therefore be specified.
In order to restrict the complication necessary for the monitoring, provision may advantageously be made for the actual value of the current temperature of the live steam to be cyclically scanned. The change in temperature and the temperature gradient may be determined by comparing successive actual values.
In a particularly advantageous embodiment, a dynamic limiting value is specified as a function of the actual value, which dynamic limiting value changes with the variation of the temperature but, as a maximum, within the compass of the maximum temperature gradient. The specification of the dynamic limiting value therefore defines a temperature range within which temperature fluctuations are permitted. This dynamic procedure takes account of permitted temperature changes, for example a continuous increase during starting. This avoids the danger of an erroneous initiation of the protective function.
Because temperature changes can occur in both directions, a lower dynamic limiting value and an upper dynamic limiting value are preferably specified. In this arrangement, the limiting values are preferably specified in such a way that they differ from the actual value by a defined temperature value. The defined temperature value therefore provides a fixed temperature range between the actual value and the upper dynamic limiting value and the lower dynamic limiting value, provided no extraordinary temperature changes occur. If, namely, temperature gradients occur which exceed the maximum permissible temperature gradient, the distance between the actual value and one of the dynamic limiting values diminishes appreciably until it finally exceeds the limiting value. The actual value curve therefore intersects the curve of the dynamic limiting value when the maximum temperature gradient is exceeded.
The fact that the dynamic limiting value has been exceeded is advantageously employed as an indication of an unallowable temperature change and the supply of the medium to the turbine is interrupted.
In order to avoid an excessively rapid initiation of the protective function, for example because of short-term electrical effects, the supply of the medium to the turbine is only interrupted, after the dynamic limiting value or the absolute limiting value has been exceeded, when the dynamic limiting value or the absolute limiting value continues to be exceeded after at least one further control scanning cycle. A certain time buffer is therefore introduced by awaiting at least one further control scanning cycle.
After the dynamic limiting value or the absolute limiting value has been exceeded, the scanning cycle is then preferably shortened, i.e. the temperature measurement is repeated at shorter intervals. In this way, the temperature scanning frequency is matched to the requirement in an advantageous manner, i.e. in the case of a normal variation, the temperature is scanned relatively seldom and, in the case of a critical variation, the temperature is scanned more frequently.
In an expedient embodiment, provision is made for the first newly measured actual value of the live steam temperature to be used to determine the dynamic limiting value in the case of a starting procedure of the turbine and/or after a fault in the monitoring of the temperature variation. This ensures a reliable mode of operation of the protective function furnished by the monitoring of the temperature change and, for example, it avoids the storage and use of the last actual value, measured before the turbine was switched off, in the determination of the dynamic limiting values. This is because this latter procedure would result, in the case of a renewed starting of the turbine, in the protective function being necessarily initiated and, therefore, the live steam supply being shut off, should the stored actual value be clearly different from the current actual value. The closing of a generator switch, in the case of a generator turbine, and the fact that there has been a departure from the minimum drive speed in the case of a driving turbine are advantageously employed as the criterion for switching on the protective function.
So that operating personnel already have an indication of a possible danger in the case of unusual temperature changes, an alarm advantageously may occur when the actual value approaches the dynamic limiting value and/or the absolute limiting value. In particular, this alarm occurs when the actual value approaches a specified distance from one of the limiting values. The alarm occurs acoustically and/or optically, for example.
In order to permit the protective function to be, as far as possible, initiated in good time, the temperature variation of the medium is monitored before the inlet of the medium to the turbine and particularly, in fact, in the region of a boiler connected upstream of the turbine or even directly after a so-called steam collecting vessel. In the case of an unallowable temperature change, therefore, the rapid shut-down takes place before the excessively cold or excessively hot steam reaches the turbine.
The protective mechanism, i.e. the possibility of preventing the supply of the medium to the turbine, can preferably only be activated when the turbine is operated below a specified load. By this means, the protective function is, in particular, not activated during the starting of the turbine. This does not impair the safety because, in this condition and in low-load operation, the danger of damage due to temperature changes is relatively small.
An object may additionally be achieved by a turbine installation, having a turbine which can be operated by a gaseous medium, having a temperature sensor for recording the temperature of the medium and having a protective device for determining the temperature variation and for interrupting the supply of the medium to the turbine when a temperature gradient is exceeded.
The advantages and expedient embodiments mentioned with respect to the method are to be transferred mutatis mutandis to the turbine installation.